The invention relates to wideband amplifiers, and more particularly to bias control circuitry operable with a compensation capacitor small enough to be integrated into a monolithic integrated circuit.
Quite a number of wideband amplifier circuits, such as those disclosed in U.S. Pat. No. 4,358,739 (Nelson) and U.S. Pat. No. 4,502,020 (Nelson et al) are known. All of the prior art wideband amplifier circuits (and also the circuit of the present invention) require input bias circuits that sense the current flowing in the push-pull gain stage to generate feedback to the input bias circuitry. In response to the feedback, the bias circuitry generates bias voltages for the input stage to prevent drift resulting from changes in output stage power dissipation. The feedback correction prevents increases in currents in the output transistors as a result of temperature-caused decreases in the V.sub.BE 's (base-to-emitter voltage) of the output transistors. Such decreased V.sub.BE 's, if not compensated for, could cause "thermal runaway" of the output transistor currents, resulting in destroying the output transistors.
The bias feedback circuits of prior wideband amplifiers always have slow response to sensed changes in the current in output transistors of the push-pull output stage. The slow response ensures that the bias circuit does not interact with the high frequency performance of the input stage of the wideband amplifier. In all prior wideband amplifiers known to us, very large compensation capacitors having capacitances of at least about 0.01 microfarads have been required to produce the needed slow feedback response. Unfortunately, it is entirely impractical to provide such a large capacitor on a monolithic or hybrid integrated circuit. Consequently, additional package pins and large external capacitors have been required in the prior integrated circuit wideband amplifiers.